Internal combustion engines are a major contributor to harmful emissions. Internal combustion engines dominate land transportation propulsion—cars, trucks, off-highway vehicles, railroad, marine, motorcycles——as well as provide mechanical and electrical power for a wide range of large and small applications. The two dominant types of internal combustion engines are spark-ignition and diesel. The amount and composition of the emissions exhausted from these engines depend on the details of the processes that occur within the engine during operation, the characteristics of the fuel used, and the type of emissions control system used.
For diesel engines, the main pollutants of concern are nitrogen oxides (NOx) and particulate matter (PM). The latter is composed of black smoke (soot), sulfates generated by the sulfur in fuel, and organic components of unburned fuel and lubricating oil.
To reduce particulate emissions, one approach is the use of diesel particulate filters (DPFs). A DPF is used downstream of the engine, in the exhaust line, and works in repeated loading and regeneration cycles. During the loading stage, the DPF filters the exhaust gas, such that PM in the exhaust is trapped inside the DPF channels while gaseous components in the exhaust pass through. During the regeneration stage, captured PM is burned off inside the DPF.
Various parameters enhance the regeneration quality of DPFs. Both exhaust gas temperature and oxygen content contribute to increased regeneration activity. Thus, regeneration occurs with the presence of oxygen, which always available in the exhaust because diesel engines operate under lean-burn conditions. Elevated temperature is provided by a burner or heater. Often, an oxidation catalyst is provided upstream of the DPF to further assist in regeneration.